This invention relates to a magneto-optical head for use in a magneto-optical reproducing device as a magneto-optical reproducing head. In the manner known in the art, it is possible to use such a head also as a magneto-optical recording and/or erasing head.
A magneto-optical recording and/or erasing device is used in recording information on a magneto-optical recording medium and/or in erasing the information for again recording new information where the information is previously erased. A magneto-optical reproducing device is used in reproducing the information. In the reproducing device, the magneto-optical reproducing head is used in combination with a laser beam source and a magneto-optical recording medium on which information is already recorded. The reproducing device is readily used also as a magneto-optical recording and/or erasing device.
Conventional magneto-optical recording and reproducing heads are described in an article contributed by Toshihisa Deguchi et al to Applied Optics, Volume 23, No. 22 (Nov. 15, 1984), pages 3972 to 3978, under the title of "Digital Magneto-optic Disk Drive" and in another article which is contributed by Masahiro Ojima et al to SPIE, Volume 529 (Optical Mass Data Storage, 1985), pages 12 to 18, and entitled "Magneto-optical Disk for Coded Data Storage." In the manner which will later be described more in detail, it is impossible with such a conventional head to avoid return to the laser beam source of a portion of a laser beam which is generated by the laser beam source and reflected from the recording medium. This gives rise to various problems.
An improved magneto-optical recording and reproducing head is disclosed in Japanese Patent Prepublication (Kokai) No. 66,537 of 1982. In the manner which will also be described in the following, the improved head comprises a first and a second beam splitter along a light path and a first arrangement for causing a forward laser beam to forwardly proceed along the light path from the laser beam source to the recording medium. When the improved head is used as a reproducing head, the recording medium responds to the forward laser beam incident thereon to produce a reversed laser beam which varies with the information recorded on the recording medium. The reversed laser beam comprises an information and an orthogonal component having polarization planes perperdicular and parallel to a polarization plane which the forward laser beam has at a point of incidence to the recording medium, respectively. The first arrangement is for furthermore causing the reversed laser beam to reversedly proceed partly along the light path from the recording medium.
The first beam splitter has a first transmissivity for a first p-polarized component, substantially totally reflects a first s-polarized component, and is azimuthally oriented as regards the light path to make the forward laser beam forwardly pass therethrough as the first p-polarized component. The second beam splitter has a second transmissivity for a second p-polarized component and substantially totally reflects a second s-polarized component. Like in the conventional head, the second beam splitter may azimuthally be oriented with respect to the light path to reflect the information component as the second s-polarized component and to make the orthogonal component reversedly pass threrethrough as the second p-polarized component.
In the improved head, a Faraday effect member is interposed between the first and the second beam splitters and used as a second arrangement for giving a 90.degree. difference between polarization planes of the forward laser beam forwardly incident thereon through the first beam splitter and of the reversed laser beam reversedly passed therethrough after having reversedly passed through the second beam splitter. Having forwardly passed through the first beam splitter and subsequently the second arrangement, the forward laser beam is made to forwardly pass through the second beam splitter as the second p-polarized component. Having reversedly passed through the second beam splitter and thereafter the second arrangement, the reversed laser beam is made to reversedly enter the first beam splitter as the first s-polarized component. As in the conventional head, an information detector may detect the information in response to the reversed laser beam reflected in the second beam splitter.
The improved head is capable of suppressing that portion of the reversed laser beam which will otherwise return to the laser beam source. It is described in the prepublication that the head has an excellent signal-to-noise ratio. The improved head, however, has a meager light utilization rate because the second transmissivity should be equal to 0.5 if the information detector is coupled to the second beam splitter as in the conventional head. When a signal producing circuit is made to produce a control signal in response to the reversed laser beam reflected in the second beam splitter like in the conventional head, the control signal is not sufficiently strong for use in servo controlling the reproducing device.